The present invention is an improvement over the flow measuring apparatus disclosed in U.S. Pat. No. 3,443,434-William C. Baker et al -- which was issued on May 13, 1969. More particularly, the aforesaid patent relates to an arrangement wherein a portion of the fluid streaming through a main supply line is diverted in a shunt path by a conduit which is electrically heated from a constant power source. A thermoelectric device is associated with the conduit, and the temperature sensed by the thermoelectric device is used to provide an indication of the flow through the shunt path and the main supply line.
The relationship that exists between fluid flow and the variables encountered in a temperature sensing arrangement for measuring fluid flow is given by the formula: EQU M .about. (H/Cp .DELTA.t)
where M is the mass flowrate, H is the heat input per unit time, Cp is the heat capacity of the fluid at constant pressure and .DELTA.t is the temperature differential between the temperature detected by a sensing device responsive to the fluid flow and a reference temperature, such as that of a heat sink.
The heat capacity term Cp cannot indiscriminately be considered a constant since it varies with fluid composition, temperature and pressure. In most fluids the variation with pressure is small and can be neglected. However, the variation with temperature can be quite large for some fluids.
The Cp for carbon dioxide, for example, varies 19.5% for a 200.degree. C. change, and such a variation is typical of the variations of the operating temperatures of some constant power flowmeters. Therefore, the accuracy of flow measurement is adversely affected by variations of Cp encountered with such a flowmeter.
Furthermore, in constant power type flowmeters a relatively large temperature change must be detected to provide good sensitivity. A typical constant power flowmeter using thermocouples to sense temperature requires a 213.degree. C. temperature differential for a 10 mvdc output.
By comparison, when operating a flowmeter in a constant temperature mode as contemplated by the present invention, variations in Cp due to flow are substantially eliminated and Cp is relatively constant for any given fluid. Additionally, it is only necessary to develop a temperature differential of sufficient magnitude to provide a stable error signal capable of maintaining the differential constant. As a result, the constant temperature flowmeter operates with improved accuracy, increased range, reduced operating temperature and improved response time.